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This study presents a rough biostratigraphy based on the planktonic foraminifers. After a comparison with the biostratigraphy of M. Filewicz, based on nannofossils, we list the relicts of the Globorotalia lineages preserved. INTRODUCTION Species of Neogloboquadrina and Globorotalia are among the most dissolution-resistant planktonic foraminifers found in sediments of the inner wall of the Middle America Trench. Despite the dissolution and slope resedimentation that occur in this seismically active area, we recognized parts of the phyletic history of these foraminifers that are useful for biostratigraphic and sedimentary interpretation. We identified a number of morphological changes in the Globorotalia group and tried to use them for local Pliocene-Pleistocene stratigraphic correlations. We believe that some of these morphological changes could be useful in paleoenvironmental studies but feel that a better knowledge of morphological trends in living globorotaliids is necessary for a better understanding. During Leg 84 of the Glomar Challenger, six sites were chosen in the Guatemala and Costa Rica active margins (Fig. 1, Table 1). In addition to contributing to our knowledge of the geological structure of the margin (Aubouin, von Huene, et al., 1982), the bathymetric distribution of five holes along an east-west transect off Guatemala, in water depths ranging from 1718 to 5529 m, offers an opportunity to study the modalities of the dissolution of planktonic foraminiferal tests at increasing water depths and the modalities of the sedimentation on the slope at that convergent extensive margin. The sites are localized Area 9 of Coulbourn, Parker, and Berger (1980), where "changes in relative abundance (of planktonic foraminiferal tests) are too subtle to define the lysocline." MATERIALS AND METHODS Approximately 700 samples were analyzed for their planktonic foraminiferal content. Of these, 358 10-cm samples were collected on board, and 366 20-cm samples were subsequently collected by the senior author at the DSDP core repository. The 20-cm samples were dried at 60° C for about 18 hr. and then weighed before being washed through series of sieves with mesh sizes of 500, 200, 125, and 50 μm, respectively. Each size fraction was again von Huene, R., Aubouin, J., et al., Init. Repts. DSDP, 84: Washington (U.S. Govt. Printing Office). 2 Address: Département de Géotectonique, Université Pierre et Marie Curie, 4, Place Jussieu, 75230 Paris Cedex 05, France. dried at 60° C and weighed; 10-cm samples were separated into the same size fractions but were not weighed. Quantitative analysis of the planktonic foraminifer assemblages was performed on the 20-cm samples for the fraction larger than 200 μm and, in some cases, for the fraction larger than 125 μm. Counts included about 1000 individuals. If the samples contained more than this number of individuals, a split of the sample was counted. Extremely few planktonic foraminifers were found on the 50-μm sieve, possibly as a result of dissolution or winnowing. The experience of Berger and Piper (1972) shows how difficult it can be to differentiate between dissolution and transport "since sediments containing only heavy resistant foraminifera may result from dissolution of less resistant tests or from bottom currents winnowing out the lighter tests." In the 10-cm samples we made a semiquantitative estimate of the abundance of the species. The sample was poured onto a tray divided in squares. Depending on the size of the studied fraction, the side of the square is 2.5 cm for the fraction larger than 500 μm, 1 cm for the fraction between 500 and 200 μm, 0.6 cm for the fraction between 200 and 125 μm, and 0.25 cm for the fraction between 125 and 50 μm. At least 10 squares were examined. The relative abundance of species or morphotypes is: r (rare)—when only 1 specimen is present in a minimum area of three squares or more; c (common)—when a specimen is present in an area of about two squares; f (frequent)—when no more than 9 individuals occur in every square; a (abundant)—when 10 to 30 individuals occur in a square; and va (very abundant)—when more than 30 individuals occur in every square. Biostratigraphy In order to place our study into a regional biostratigraphic framework we compiled biostratigraphic studies in the East Pacific calibrated by various authors on the paleomagnetic time scale for the early Miocene to Recent (Figs. 2 and 3). For our geological time scale we chose the dates of the paleomagnetic anomalies calculated by Lowrie and Alvarez (1981). For the middle and early Miocene the calibration on the paleomagnetic time scale is indirect, and some discrepancies still exist, chiefly for the Langhian and Serravallian stages. Among the biostratigraphic studies of various microfossil groups conducted-on Leg 84 material, only that of the calcareous nannofossils (Filewicz, this volume) was available to us at the time of this study; we use Filewicz's nannozonation extensively to calibrate coiling changes in Pulleniatina and Globorotalia found in our samples. Because we need to place the Globorotalia study in a planktonic foraminiferal framework, we examined all the planktonic foraminiferal assemblages. Results of our observations are given as range charts (Tables 2-12) and summarized (Figs. 4-8). Site by site discussion occurs after the following brief examination of dissolution effects. Dissolution Analysis Results of counts are summarized in Tables 2 to 12. Evidence of dissolution of the planktonic foraminiferal tests, as illustrated by Be, Morse, and Harrison (1975), is present throughout the cores. Degree of dissolution ranges from shells that are partly abraded, to fragmented, to totally eliminated. Site 570 off Guatemala, is the shallowest site, with a water depth of about 1700 m; Cores 570-1 to -21 exhibit

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Upper Oligocene to Recent Planktonic Foraminiferal Remains in Sediments of the Inner Wall of the Middle America Trench with Special Emphasis on Globorotalia